1. Field of the Invention
The invention relates, for example, in an image forming apparatus, to a heating apparatus suitable for use as an image heating apparatus for fixing an unfixed image formed and borne on a recording material, and particularly to a heating apparatus of an electromagnetic (magnetic) induction heating type.
2. Description of Related Art
Description will hereinafter be made with an image heating and fixing apparatus in an image forming apparatus such as an electrophotographic copying machine, a printer or a facsimile apparatus taken as an example.
The image heating and fixing apparatus in the image forming apparatus is an apparatus in which an unfixed toner image formed on the surface of a recording material by a direct process or an indirect (transfer) process is heated and fixed as a permanently secured image on the surface of the recording material in the image forming portion of the image forming portion by suitable image forming process means such as electrophotography, electrostatic recording or magnetic flux recording by the use of a toner (visualizing agent) comprising heat-soluble resin or the like.
There has heretofore been an electromagnetic inductive heating process as the heating process of such an image heating and fixing apparatus. This is an apparatus which uses an electromagnetic inductive heat generating member as a heating member, and causes a magnetic field to act on the electromagnetic inductive heat generating member by magnetic field generating means to thereby impact heat to the recording material as a heated material by joule heat generation based on an eddy current generated in the electromagnetic induction heat generating member, and heat and fix an unfixed toner image on the surface of the recording material.
In Japanese Patent Publication (Koukoku) No. 5-9027 B, there is disclosed an apparatus of a heat roller type in which a fixing roller of a ferromagnetic material is electromagnetically induction-heated, and this apparatus enables a heat generating position to be near to a fixing nip portion, and achieves a fixing process higher in efficiency than an apparatus of a heat roller type using a halogen lamp as a heat source.
This apparatus, however, in great in the heat capacity of the fixing roller and has therefore suffered from the problem that to raise the temperature of the fixing nip portion by limited electric power, great electric power is required.
In Japanese Patent Application Laid-Open No. 4-166966, there is disclosed a fixing apparatus of an electromagnetic inductive heating type using a film-shaped fixing roller reduced in heat capacity.
In the film-shaped fixing roller reduced in heat capacity, however, a heat flow in the longitudinal direction thereof (the lengthwise direction of the fixing nip portion thereof) is impeded and therefore, when a recording material of a small size is passed, an excess temperature rise in a non-paper passing portion (temperature rise of the non-paper passing portion) occurs, and this has given rise to the problem that the life of film or a pressure roller is reduced. This problem of the temperature rise of the non-paper passing portion also holds true in the case of an apparatus of a film heating type.
In Japanese Patent Application Laid-Open No. 10-74009, there is disclosed a heating apparatus characterized by magnetic flux adjusting means for varying the density distribution of an acting magnetic flux with respect to the lengthwise direction of a fixing roller (film). By this fixing apparatus of an electromagnetic inductive heating type, there has been shown a method of solving the temperature rise of the non-paper of solving the temperature rise of the non-paper passing portion. Also, there is disclosed means for moving the magnetic flux adjusting means by predetermined driving means such as a motor or a solenoid, and effecting the adjustment of the magnetic flux of the non-paper passing portion of the fixing roller (film).
Also, in Japanese Patent Application Laid-Open No. 09-171889, it is disclosed to movably provide a magnetic flux shield plate on the inner surface of the cylindrical film guide member of fixing film.
An image forming apparatus using a heating apparatus of the well-known electromagnetic inductive heating type as a fixing apparatus as described above suffers from the following problems.
Magnetic field generating means generates an alternating magnetic flux by an alternating current supplied thereto. In Japanese Patent Application Laid-Open No. 10-74009, magnetic field generating means and magnetic flux shield means are disposed so as to have a clearance therebetween. This has led to the problem that when this alternating magnetic flux acts on the magnetic flux shield means, a repulsive force is born between the magnetic flux shield means and the magnetic field generating means, and the magnetic flux shield means is vibrated to thereby produce a periodic vibration sound.
Also, in a construction wherein the magnetic flux shield means is moved in the interior of the fixing film as in Japanese Patent Application Laid-Open No. 10-74009 and Japanese Patent Application Laid-Open No. 09-171889, the magnetic flux shield means is provided in such a manner as to be along the inner surface of a pressure member (film holder) brought into contact with the fixing film, and this has led to the problem that a fixing pressure force is applied to the holder, whereby the holder is deformed, thus giving rise to the faulty operation of the magnetic flux shield means. This faulty operation of the shield means in turn has led to the problem that the generated heat distribution of an induction heat generating member in a direction orthogonal to the conveying direction of a material to be heated cannot be appropriately controlled and the abnormal temperature rise of the non-paper passing portion is caused. Also, a fixing pressure member (holder) slides in contact with the film and therefore, there has been a problem from the viewpoint of durability.